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We applied serological analysis of recombinant cDNA expression libraries (SEREX) to cases of esophageal squamous cell carcinoma (SCC) to identify tumor antigens. One of the clones identified was TROP2, which is known as calcium signal transducer. To evaluate the clinical significance of serum anti-TROP2 antibodies (s-TROP2-Abs) in patients with esophageal SCC, the presence of s-TROP2-Abs was analyzed by Western blotting using bacterially expressed TROP2 protein. We found that 23 of 75 (31%) patients were positive for s-TROP2-Abs. Positivity in terms of s-TROP2-Abs showed a significant association with tumor size but not with other clinicopathological features. The protein expression levels of TROP2 were much higher in esophageal SCC cell lines as compared to those in normal esophageal mucosa and its immortalized cells although the mRNA expression levels were not necessarily elevated in malignant cell lines and tissues. Immunohistochemical studies showed that the expression of TROP2 protein in esophageal SCC specimens was noticeably higher than that found in mild hyperplasia of esophageal mucosae. Thus, s-TROP2-Abs seemed useful in the diagnosis of SCC and may be a candidate for serum tumor markers.

We have developed a method that we call 'drug-sensitivity pattern analysis', or DSPA, for analysis of protein function. Cells are transfected with cDNA of the test molecule, followed by analysis of the sensitivity of the transfected cells to multiple growth-inhibitory drugs. If two cDNA products have similar functions, their transfected cells should show similar drug-sensitivity patterns. The cDNAs of some signaling molecules were transfected into NIH3T3 or Ha-ras-transformed NIH3T3 (ras-NIH) cells and stable transfectants, which expressed high amounts of the gene product, were isolated. Chemosensitivity of the transfected clone was compared with the parental cells by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide method using more than 40 drugs. The chemosensitivity changes caused by the transfected gene were calculated and expressed numerically as 'drug chemosensitivity index' (DCI). When the DCI values were analyzed by regression analysis, a significant positive relationship between IkappaBalpha superrepressor and dominant-negative IKKbeta and an inverse relationship between p53 and Mdm2 were consistent with previous reports. Thus, the DSPA method is useful for identifying functional similarities between gene products.

Thymidine kinase (TK) is a protein closely associated with DNA replication. Here we show that, in contrast with the variation of TK activity, which changed parallel with S phase cell distribution during asynchronous culture of rat hepatoma JB1 cells, the serine residues of cytosolic TK protein was phosphorylated continuously in response to increasing G0/G1 phase cell distribution. The shifting of phosphorylation of TK protein during different cell growth conditions was further confirmed with the examination of the elution profile of cytosolic TK activity by anion-exchange high performance liquid chromatography (HPLC). HPLC analysis revealed that the rapid proliferating (62 h after asynchronous culture) rat hepatoma JB1 cells showed only the hyperphosphorylated form of cytosolic TK activity, while synchronously M phase-arrested JB1 cells showed hypo- and hyper-phosphorylated forms of cytosolic TK activity. These results suggested that the modulation of phosphorylation of cytosolic TK protein was cell cycle-dependent, and that the phosphorylation of cytosolic TK protein might be involved in the negative regulation of TK activity in vivo.

Arginase in salivary glands is potentially involved in the synthesis of proline, glutamate, and polyamines that play specific physiological roles in the glands, and also in depletion of arginine in the oral cavity to protect teeth from microorganisms. We detected protein and mRNA for the type I isoform of arginase in mouse salivary glands. Enzymes of the arginine-biosynthetic pathway were also detected. Immunohistochemical analysis revealed that arginase I was enriched in the striated duct, and was also present in the acinus, demilune and granulated duct. Mice with targeted disruption of the gene for C/EBPalpha, which is a transcription factor essential for expression of the arginase I gene in the liver, showed dramatically reduced immunoreactivity for arginase I in the parotid gland but not in the submandibular and sublingual glands. Therefore, C/EBPalpha is specifically required for expression of the arginase I gene in the parotid gland.

Anti-TK monoclonal antibodies (mAbs) were raised against recombinant human cytosolic thymidine kinase (rhTK) and characterized by Western immunoblotting, enzyme-linked immunosorbent assay (ELISA) and immunostaining of tumor cells. Twenty-three clones of TK mAbs were characterized to recognize specifically not only rhTK produced by Escherichia coli but also TK subunit of 25 kDa in human lung cancer. The anti-TK mAbs reacted specifically with cytosolic TK but not with mitochondrial TK. Only one clone of the mAbs inhibited the catalytic activity of TK. By solid phase sandwich enzyme immunoassay using these mAbs, we could quantitate the cytosolic TK content in tissues. Immunohistochemical staining analysis using one of the TK mAbs showed that human lung adenocarcinoma and squamous cell carcinoma exhibited much higher staining intensity than stromal cells. These mAbs are useful for biochemical studies on the regulation of human TK in proliferating cells such as tumor cells and for diagnosis of highly proliferating tumors.

The cDNA clone encoding human thymidine kinase (hTK), was expressed in E. coli using a prokaryotic expression vector, pKK 223-3. The kinetics of the recombinant hTK (rhTK) were similar to those of cytosolic TK but not of mitochondrial TK. rhTK was highly purified in the presence of either ATP or dithiothreitol (DTT). The specific activity of rhTK purified in the presence of ATP [rhTK(ATP)] was lower than that of rhTK purified in the presence of DTT [rhTK(DTT)]. Activity of the purified rhTK(ATP) was enhanced by addition of thiols including DTT, cysteine, homocysteine and beta-mercaptoethanol but inhibited by various sulfhydryl reagents such as 5,5'-dithio-bis(2-nitrobenzoic acid). Hence, it was suggested that rhTK is a thiol-type enzyme. Apparent Mr of purified rhTK(ATP) was 100 kDa, which corresponds to the size of a tetramer (25 kDa subunit), while that of purified rhTK(DTT) was 50 kDa, the size of a dimer. The tetramer form of rhTK(ATP) was converted to the dimer by replacement of ATP by DTT. On the other hand, the dimer form of rhTK(DTT) was converted to the tetramer by addition of ATP. Thus, the catalytic activity of human cytosolic TK might be regulated by thiols as well as ATP via its polymerization status.

Summary Cytosolic thymidine kinase was purified 18,000-fold of the homogenate from murine Ehrlich ascites tumor, using the [p-aminophenyl 3'-dTMP]-CH-Sepharose affinity column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band of molecular weight 26,000. Two different forms, relative molecular weight 50,000 and 70,000, were found by gel f'dtration, depending on the existence of dithiothreitol, ATP and other nucleotides. These agents also stabilize and stimulate the enzyme activity. The existence of two forms was.also manifested by DEAE-Sephacel column chromatography, where the 50,000 form was eluted by 50 mM NaC1 and the 70,000 form by 400 mM NaC1.

It was revealed that thymidylate kinase was purified together with cytosolic thymidine kinase from human term placenta by p-aminophenyl thymidine-3'-phosphate-CH-Sepharose affinity column chromatography, which has been commonly used for purification of thymidine kinase. In addition, it was noted that mitochondrial thymidine kinase and nucleoside diphosphate kinase were concurrently eliminated. In the presence of ATP, cytosolic thymidine kinase and thymidylate kinase could be separated from each other by Ultrogel AcA 34 filtration, and their molecular weights were estimated to be 70,000 and 50,000, respectively. On SDS-polyacrylamide gel electrophoresis, thymidine kinase protein exhibited a band of 26,000, which was compatible with the molecular weight of the enzyme subunit calculated from its cDNA, while thymidylate kinase protein showed 24,000. Thymidylate kinase could utilize either ATP or dATP as an efficient phosphate donor, and showed substrate specificity for dTMP.